Sources of fossil fuels useful for heating, transportation, and the production of chemicals as well as petrochemicals are becoming increasingly more scarce and costly. Industries such as those producing energy and petrochemicals are actively searching for cost effective engineered fuel feed stock alternatives for use in generating those products and many others. Additionally, due to the ever increasing costs of fossil fuels, transportation costs for moving engineered fuel feed stocks for production of energy and petrochemicals is rapidly escalating.
These energy and petrochemical producing industries, and others, have relied on the use of fossil fuels, such as coal and oil and natural gas, for use in combustion and gasification processes for the production of energy, for heating and electricity, and the generation of synthesis gas used for the downstream production of chemicals and liquid fuels, as well as an energy source for turbines.
Combustion and gasification are thermochemical processes that are used to release the energy stored within the fuel source. Combustion takes place in a reactor in the presence of excess air, or excess oxygen. Combustion is generally used for generating steam which is used to power turbines for producing electricity. However, the brute force nature of the combustion of fuel causes significant amounts of pollutants to be generated in the gas produced. For example, combustion in an oxidizing atmosphere of, for example, fossil fuels such as coal, oil and natural gas, releases nitrogen oxides, a precursor to ground level ozone which can stimulate asthma attacks. Combustion is also the largest source of sulfur dioxide which in turn produces sulfates that are very fine particulates. Fine particle pollution from U.S. coal fired power plants cuts short the lives of over 30,000 people each year. Hundreds of thousands of Americans suffer from asthma attacks, cardiac problems and upper and lower respiratory problems associated with fine particles from power plants.
Effect of Coal Burning on the Environment The world's power demands are expected to rise 60% by 2030 (http://www.iea.org/textbase/nppdf/free/2004/weo2004.pdf). With the worldwide total of active coal plants over 50,000 and rising, the International Energy Agency (IEA) estimates that fossil fuels will still account for 85% of the energy market by 2030 (http://www.sciencedaily.com/releases/2007/11/071114163448.htm). World organizations, and international agencies like the IEA are concerned about the environmental impact of burning fossil fuels. Coal power stations are the least carbon efficient power stations in terms of the level of carbon dioxide produced per unit of electricity generated. The combustion of fossil fuels contributes to acid rain, global warming, and air pollution due to the impurities and chemical composition of the fuel (electricity generation is responsible for 41% of U.S. manmade carbon dioxide emissions) (http://www.epa.gov/climatechange/ emissions/co2_human.html). Acid rain is caused by the emission of nitrogen oxides and sulfur dioxide into the air. These themselves may be only mildly acidic, yet when it reacts with the atmosphere, it creates acidic compounds such as sulfurous acid, nitric acid and sulfuric acid that fall as rain, hence the term acid rain. In Europe and the U.S., stricter emission laws have reduced the environmental hazards associated with this problem.
Coal also contains low levels of uranium, thorium, and other naturally-occurring radioactive isotopes whose release into the environment leads to radioactive contamination. While these substances are present as very small trace impurities, enough coal is burned that significant amounts of these substances are released. A 1,000 MW coal-burning power plant could release as much as 5.2 tons/year of uranium (containing 74 pounds of uranium-235) and 12.8 tons/year of thorium. The radioactive emission from this coal power plant is 100 times greater than a comparable nuclear power plant with the same electrical output; including processing output, the coal power plant's radiation output is over 3 times greater (http://www.ornl.gov/info/ornlreview/rev26-34/text/colmain.html).
Trace amounts of mercury exist in coal and other fossil fuels (http://www.fossil.energy.gov/programs/powersystems/pollutioncontrols/overview_mercurycontrols.html). When these fuels burn, toxic mercury is released which accumulates in food chains and is especially harmful to aquatic ecosystems. The worldwide emission of mercury from both natural and human sources was an estimated 5,500 tons in 1995. U.S. coal-fired plants emit an estimated 48 tons of mercury annually, which is approximately ⅓ of all mercury emitted into the air by human activity in the U.S.
Gasification also takes place in a reactor, although in the absence of air, or in the presence of substoichiometric amounts of oxygen. The thermochemical reactions that take place in the absence of oxygen or under substoichiometric amounts of oxygen do not result in the formation of nitrogen oxides or sulfur oxides. Therefore, gasification can eliminate much of the pollutants formed during the firing of fuels, especially sulfur containing fuels such as for example coal.
Gasification generates a gaseous, fuel rich product known as synthesis gas (syngas). During gasification, two processes take place that convert the fuel source into a useable fuel gas. In the first stage, pyrolysis releases the volatile components of the fuel at temperatures below 600° C. (1112° F.), a process known as devolatization. The pyrolysis also produces char that consists mainly of carbon or charcoal and ash. In the second gasification stage, the carbon remaining after pyrolysis is either reacted with steam, hydrogen, or pure oxygen. Gasification with pure oxygen results in a high quality mixture of carbon monoxide and hydrogen due to no dilution of nitrogen from air.
A variety of gasifier types have been developed. They can be grouped into four major classifications: fixed-bed updraft, fixed-bed downdraft, bubbling fluidized-bed and circulating fluidized bed. Differentiation is based on the means of supporting the fuel source in the reactor vessel, the direction of flow of both the fuel and oxidant, and the way heat is supplied to the reactor. The advantages and disadvantages of these gasifier designs have been well documented in literature, for example, Rezaiyan, J. and Nicholas P. Cheremisinoff, Gasification Technology, A Primer for Engineers and Scientists. Boca Raton: CRC Press, 2005, the contents of which are hereby incorporated by reference.
The updraft gasifier, also known as counterflow gasification, is the oldest and simplest form of gasifier; it is still used for coal gasification. The fuel is introduced at the top of the reactor, and a grate at the bottom of the reactor supports the reacting bed. The oxidant in the form of air or oxygen and/or steam are introduced below the grate and flow up through the bed of fuel and char. Complete combustion of char takes place at the bottom of the bed, liberating CO2 and H2O. These hot gases (˜1000° C.) pass through the bed above, where they are reduced to H2 and CO and cooled to about 750° C. Continuing up the reactor, the reducing gases (H2 and CO) pyrolyse the descending dry fuel and finally dry any incoming wet fuel, leaving the reactor at a low temperature (˜500° C.). Updraft gasification is a simple, low cost process that is able to handle fuel with a high moisture and high inorganic content. The primary disadvantage of updraft gasification is that the synthesis gas contains 10-20% tar by weight, requiring extensive syngas cleanup before engine, turbine or synthesis applications.
Downdraft gasification, also known as concurrent-flow gasification, has the same mechanical configuration as the updraft gasifier except that the oxidant and product gases flow down the reactor, in the same direction as the fuel, and can combust up to 99.9% of the tars formed. Low moisture fuel (<20%) and air or oxygen are ignited in the reaction zone at the top of the reactor, generating pyrolysis gas/vapor, which burns intensely leaving 5 to 15% char and hot combustion gas. These gases flow downward and react with the char at 800 to 1200° C., generating more CO and H2 while being cooled to below 800° C. Finally, unconverted char and ash pass through the bottom of the grate and are sent to disposal. The advantages of downdraft gasification are that up to 99.9% of the tar formed is consumed, requiring minimal or no tar cleanup. Minerals remain with the char/ash, reducing the need for a cyclone. The disadvantages of downdraft gasification are that it requires feed drying to a low moisture content (<20%). The syngas exiting the reactor is at high temperature, requiring a secondary heat recovery system; and 4-7% of the carbon remains unconverted.
The bubbling fluidized bed consists of fine, inert particles of sand or alumina, which have been selected for size, density, and thermal characteristics. As gas (oxygen, air or steam) is forced through the inert particles, a point is reached when the frictional force between the particles and the gas counterbalances the weight of the solids. At this gas velocity (minimum fluidization), the solid particles become suspended, and bubbling and channeling of gas through the media may occur, such that the particles remain in the reactor and appear to be in a “boiling state”. The minimum fluidization velocity is not equal to the minimum bubbling velocity and channeling velocity. For coarse particles, the minimum bubbling velocity and channeling velocity are close or almost equal, but the channeling velocity may be quite different, due to the gas distribution problem. The fluidized particles tend to break up the fuel fed to the bed and ensure good heat transfer throughout the reactor. The advantages of bubbling fluidized-bed gasification are that it yields a uniform product gas and exhibits a nearly uniform temperature distribution throughout the reactor. It is also able to accept a wide range of fuel particle sizes, including fines; provides high rates of heat transfer between inert material, fuel and gas.
The circulating fluidized bed gasifiers operate at gas velocities higher than the so-called transport velocity or onset velocity of circulating fluidization at which the entrainment of the bed particles dramatically increases so that continuous feeding or recycling back the entrained particles to the bed is required to maintain a stable gas-solid system in the bed.—The circulating fluidized-bed gasification is suitable for rapid reactions offering high heat transport rates due to high heat capacity of the bed material. High conversion rates are possible with low tar and unconverted carbon.
Normally these gasifiers use a homogeneous source of fuel. A constant unchanging fuel source allows the gasifier to be calibrated to consistently form the desired product. Each type of gasifier will operate satisfactorily with respect to stability, gas quality, efficiency and pressure losses only within certain ranges of the fuel properties. Some of the properties of fuel to consider are energy content, moisture content, volatile matter, ash content and ash chemical composition, reactivity, size and size distribution, bulk density, and charring properties. Before choosing a gasifier for any individual fuel it is important to ensure that the fuel meets the requirements of the gasifier or that it can be treated to meet these requirements. Practical tests are needed if the fuel has not previously been successfully gasified.
Normally, gasifiers use a homogeneous source of fuel for producing synthesis gas. A constant unchanging fuel source allows the gasifier to be calibrated to consistently form the desired product. Each type of gasifier will operate satisfactorily with respect to stability, gas quality, efficiency and pressure losses only within certain ranges of the fuel properties. Some of the properties of fuel to consider for combustion and gasification are high heating value (HHV) content, carbon (C), hydrogen (H), and oxygen (O) content, BTU value, moisture content, volatile matter content, ash content and ash chemical composition, sulfur content, chlorine content, reactivity, size and size distribution, and bulk density. Before choosing a gasifier for any individual fuel it is important to ensure that the fuel meets the requirements of the gasifier or that it can be treated to meet these requirements. Practical tests are needed if the fuel has not previously been successfully gasified.
One potential source for a large amount of feed stock for combustion and gasification is waste. Waste, such as municipal solid waste (MSW), is typically disposed of or used in combustion processes to generate heat and/or steam for use in turbines. The drawbacks accompanying combustion have been described above, and include the production of pollutants such as nitrogen oxides, sulfur oxide, particulates and products of chlorine that damage the environment.
Effect of Greenhouse Gases Arising from Coal Burning on the Environment
One of the most significant threats facing the environment today is the release of pollutants and greenhouse gases (GHGs) into the atmosphere through the combustion of fuels such as coal. GHGs such as carbon dioxide, methane, nitrous oxide, water vapor, carbon monoxide, nitrogen oxide, nitrogen dioxide, and ozone, absorb heat from incoming solar radiation but do not allow long-wave radiation to reflect back into space. GHGs in the atmosphere result in the trapping of absorbed heat and warming of the earth's surface. In the U.S., GHG emissions come mostly from energy use driven largely by economic growth, fuel used for electricity generation, and weather patterns affecting heating and cooling needs. Energy-related carbon dioxide emissions, resulting from petroleum and natural gas, represent 82 percent of total U.S. human-made GHG emissions. Another greenhouse gas, methane, comes from landfills, coal mines, oil and gas operations, and agriculture; it represents nine percent of total emissions. Nitrous oxide (5 percent of total emissions), meanwhile, is emitted from burning fossil fuels and through the use of certain fertilizers and industrial processes. World carbon dioxide emissions are expected to increase by 1.9 percent annually between 2001 and 2025. Much of the increase in these emissions is expected to occur in the developing world where emerging economies, such as China and India, fuel economic development with fossil energy. Developing countries' emissions are expected to grow above the world average at 2.7 percent annually between 2001 and 2025; and surpass emissions of industrialized countries near 2018.
Waste landfills are also significant sources of GHG emissions, mostly because of methane released during decomposition of waste, such as, for example, MSW. Compared with carbon dioxide, methane is twenty-times stronger than carbon dioxide as a GHG, and landfills are responsible for about 4% of the anthropogenic emissions. Considerable reductions in methane emissions can be achieved by combustion of waste and by collecting methane from landfills. The methane collected from the landfill can either be used directly in energy production or flared off, i.e., eliminated through combustion without energy production (Combustion Of Waste May Reduce Greenhouse Gas Emissions, ScienceDaily, Dec. 8, 2007).
One measure of the impact human activities have on the environment in terms of the amount of green house gases produced is the carbon footprint, measured in units of carbon dioxide (CO2). The carbon footprint can be seen as the total amount of carbon dioxide and other GHGs emitted over the full life cycle of a product or service. Normally, a carbon footprint is usually expressed as a CO2 equivalent (usually in kilograms or tons), which accounts for the same global warming effects of different GHGs. Carbon footprints can be calculated using a Life Cycle Assessment method, or can be restricted to the immediately attributable emissions from energy use of fossil fuels.
An alternative definition of carbon footprint is the total amount of CO2 attributable to the actions of an individual (mainly through their energy use) over a period of one year. This definition underlies the personal carbon calculators. The term owes its origins to the idea that a footprint is what has been left behind as a result of the individual's activities. Carbon footprints can either consider only direct emissions (typically from energy used in the home and in transport, including travel by cars, airplanes, rail and other public transport), or can also include indirect emissions which include CO2 emissions as a result of goods and services consumed, along with the concomitant waste produced.
The carbon footprint can be efficiently and effectively reduced by applying the following steps: (i) life cycle assessment to accurately determine the current carbon footprint; (ii) identification of hot-spots in terms of energy consumption and associated CO2-emissions; (iii) optimization of energy efficiency and, thus, reduction of CO2-emissions and reduction of other GHG emissions contributed from production processes; and (iv) identification of solutions to neutralize the CO2 emissions that cannot be eliminated by energy saving measures. The last step includes carbon offsetting, and investment in projects that aim at the reducing CO2 emissions.
The purchase of carbon offsets is another way to reduce a carbon footprint. One carbon offset represents the reduction of one ton of CO2-eq. Companies that sell carbon offsets invest in projects such as renewable energy research, agricultural and landfill gas capture, and tree-planting.
Purchase and withdrawal of emissions trading credits also occur, which creates a connection between the voluntary and regulated carbon markets. Emissions trading schemes provide a financial incentive for organizations and corporations to reduce their carbon footprint. Such schemes exist under cap-and-trade systems, where the total carbon emissions for a particular country, region, or sector are capped at a certain value, and organizations are issued permits to emit a fraction of the total emissions. Organizations that emit less carbon than their emission target can then sell their “excess” carbon emissions.
For many wastes, the disposed materials represent what is left over after a long series of steps including: (i) extraction and processing of raw materials; (ii) manufacture of products; (iii) transportation of materials and products to markets; (iv) use by consumers; and (v) waste management. At virtually every step along this “life cycle,” the potential exists for greenhouse gas (GHG) impacts. Waste management affects GHGs by affecting energy consumption (specifically, combustion of fossil fuels) associated with making, transporting, using, and disposing the product or material that becomes a waste and emissions from the waste in landfills where the waste is disposed.
Incineration typically reduces the volume of the MSW by about 90% with the remaining 10% of the volume of the original MSW still needing to be landfilled. This incineration process produces large quantities of the GHG CO2. Typically, the amount of energy produced per equivalents CO2 expelled during incineration are very low, thus making incineration of MSW for energy production one of the worst offenders in producing GHG released into the atmosphere. Therefore, if GHGs are to be avoided, new solutions for the disposal of wastes, such as MSW, other than landfilling and incineration, are needed.
Each material disposed of as waste has a different GHG impact depending on how it is made and disposed. The most important GHGs for waste management options are carbon dioxide, methane, nitrous oxide, and perfluorocarbons. Of these, carbon dioxide (CO2) is by far the most common GHG emitted in the US. Most carbon dioxide emissions result from energy use, particularly fossil fuel combustion. Carbon dioxide is the reference gas for measurement of the heat-trapping potential (also known as global warming potential or GWP). By definition, the GWP of one kilogram (kg) of carbon dioxide is 1. Methane has a GWP of 21, meaning that one kg of methane has the same heat-trapping potential as 21 kg of CO2. Nitrous oxide has a GWP of 310. Perfluorocarbons are the most potent GHGs with GWPs of 6,500 for CF4 and 9,200 for C2F6. Emissions of carbon dioxide, methane, nitrous oxide, and perfluorocarbons are usually expressed in “carbon equivalents.” Because CO2 is 12/44 carbon by weight, one metric ton of CO2 is equal to 12/44 or 0.27 metric tons of carbon equivalent (MTCE). The MTCE value for one metric ton of each of the other gases is determined by multiplying its GWP by a factor of 12/44 (The Intergovernmental Panel on Climate Change (IPCC), Climate Change 1995: The Science of Climate Change, 1996, p. 121). Methane (CH4), a more potent GHG, is produced when organic waste decomposes in an oxygen free (anaerobic) environment, such as a landfill. Methane from landfills is the largest source of methane in the US.
The greater GHG emission reductions are usually obtained when recycled waste materials are processed and used to replace fossil fuels. If the replaced material is biogenic (material derived from living organisms), it is not always possible to obtain reductions of emissions. Even other factors, such as the treatment of the waste material and the fate of the products after the use, affect the emissions balance. For example, the recycling of oil-absorbing sheets made of recycled textiles lead to emission reductions compared with the use of virgin plastic. In another example, the use of recycled plastic as raw material for construction material was found to be better than the use of impregnated wood. This is because the combustion of plastic causes more emissions than impregnated wood for reducing emissions. If the replaced material had been fossil fuel-based, or concrete, or steel, the result would probably have been more favorable to the recycling of plastic.
Given the effect of GHGs on the environment, different levels of government are considering, and in some instances have initiated, programs aimed at reducing the GHGs released into the atmosphere during the conversion of fuels into energy. One such initiative is the Regional Greenhouse Gas Initiative (RGGI). RGGI is a market-based program designed to reduce global warming pollution from electric power plants in the Northeast. Other such initiatives are being considered in different sections of the U.S. and on the federal level. RGGI is a government mandated GHG trading system in the Northeastern U.S. This program will require, for example, that coal-fired power plants aggressively reduce their GHG emissions by on average 2.5% per year. One way to do this is by changing the fuel source used or scrubbing the emissions to remove the pollutants. An alternative is to purchase carbon credits generated by others which can offset their emissions into the atmosphere.
Other emissions to be avoided are sulfur emissions as well as chlorine emissions. Fuels and waste containing significant amounts of sulfur or chlorine should be avoided for combustion and gasification reactions. Significant amounts are defined as an amount that when added to a final fuel feed stock causes the final feed stock to have more than 2% sulfur or more than 1% of chlorine. Materials such as coal, used tires, carpet, and rubber, when combusted, release unacceptable amounts of harmful sulfur- and chlorine-based gases.
Thus, there is a need for alternative fuels that burn efficiently and cleanly and that can be used for the production of energy and/or chemicals. There is at the same time a need for waste management systems that implement methods for reducing GHG emissions of waste by utilizing such wastes. In particular, there is a need for reducing the carbon foot print of materials by affecting their end-stage life cycle management. By harnessing and using the energy content contained in waste, it is possible to reduce GHG emissions generated during the processing of wastes and effectively use the waste generated by commercial and residential consumers.
It is an object of the present invention to provide an engineered fuel feed stock (EF) containing specified chemical molecular characteristics, such as carbon content, hydrogen content, oxygen content, sulfur content, ash content, moisture content, and HHV for thermal-conversion of carbon-containing materials, such as coal. The engineered fuel feed stock described here is useful for many purposes including, but not limited to, offsetting or replacing coal as the feed stock in coal-fired power generating plants. It is also an object of this invention to provide novel feed stocks, which when combusted produce a better emission profile as compared to combusted coal.